Microwave plasmas are used in the industrial chemical processing of gases. This is typically accomplished by flowing the gases to be reacted through an elongated vessel while microwave energy is coupled into the vessel to generate a plasma. The plasma cracks the gas molecules into component species. Microwave chemical processing systems are effective because microwave plasmas operate at relatively high power coupling efficiencies at low ion energies, and are capable of supporting various gas reactions, such as the conversion of methane into hydrogen and carbon particulates, the conversion of carbon dioxide into oxygen and carbon, and coating particulates and other seed materials with other layers for functionalization and complex layered materials and aggregates processing.
Typical systems for chemical gas processing include a quartz reaction chamber through which process materials flow, and a microwave magnetron source coupled to the reaction chamber through a waveguide. The input microwave energy can be continuous wave or pulsed. Systems are designed to control the effective coupling of the microwave energy into the reaction chamber, and the gas flow within the reaction chamber to improve the energy absorption by the flowing gas. Often the systems include a wedge located where the microwave waveguide intersects the quartz reaction chamber, to concentrate the electric field within a small area, and the waveguide conductive walls are not exposed to the gases to be processed.
In microwave chemical processing systems that produce gases and particulates, the particulate filtration is accomplished using a gas-solids separation system. The gas-solids separation systems can contain cyclone filters, back-pulse filters, or other filters. For example, filtering the carbon-containing particles from the hydrogen gas that are generated in microwave chemical processing systems is challenging. In some cases, the generated carbon-containing particles are very small (e.g., median particle size below 100 nm), which exacerbates the particle filtration challenges. Some gas-solids separation systems for separating carbon-containing particles from a gas stream use back-pulse filters. In some cases, the back-pulse filters employ heated filters (e.g., heated filter candles). In some of these systems, the back-pulse filters are periodically cleared by blowing gas through the filter candles to dislodge carbon-containing particles (i.e., using a back-pulse that flows gas in the opposite direction the from the filtration direction). Other gas-solids separation systems for separating carbon-containing particles from hydrogen gas use cyclone separators. In some cases, the cyclone separators are also heated.